Recently, with a request for reduction in a size and reduction in a thickness of a power circuit component, a chip inductor having a laminate structure is developed and put into use as a transformer or a choke coil used in a power supply circuit such as a DC-DC converter.
In such a multilayer inductor, electrically insulating magnetic layers and conductive body patterns are alternately laminated, and the above conductive body patterns are connected in the lamination direction in sequence, so that a coil that spirally circulates while being superimposed in the lamination direction is formed in a magnetic body, and each of the both ends of the coil is drawn out on the outer surface of a laminated body chip through a draw-out conductor. Herein, a ferrite is used as the magnetic body, the magnetic layers and the conductive body patterns are formed to be laminated by using, for example, a screen printing method.
On the other hand, in a mobile market where the reduction in a size is recently requested, a current value which flows in an inductor increases in accordance with rise in the switching frequency of a power supply to be used, and improvement in processing performance of the power supply. In the above ferrite, a loss at a high frequency (several MHz to several dozens MHz) is generally less, and therefore a laminated chip inductor using a ferrite material is optimized for a mobile power supply that operates at a high switching frequency. Additionally, in the mobile market, since the chip shape is excellent in mounting performance or mass productivity, a large number of laminated chip inductors have been employed.
However, the above ferrite generally tends to be low in magnetic flux saturation density, and to be bad in DC superposition characteristics, and therefore it is becoming difficult to follow the electric current increase in the recent mobile market.
In order to solve this, a measure for improving the DC superposition characteristics in the above multilayer inductor by increasing the size of the above coil, and reducing the density of a magnetic flux that flows in the coil, or using a metal material unlikely to be saturated as a magnetic material itself is considered. However, when the above coil size is increased, increase in the whole of the multilayer inductor is caused, which is against the market request. Additionally, a chip inductor, in which the chip shape having excellent mounting performance is maintained, and in which a metal material unlikely to cause magnetic saturation is used as a magnetic body, appears in the market. However, the chip inductor has disadvantage that the metal material generally has a large loss at a high frequency compared to the ferrite, and that conversion efficiency is reduced in a conversion usage.
The magnetic body used for the above multilayer inductor is saturated by a magnetic flux excited from a current flowing in the coil in the operation of a power supply. Accordingly, when the saturation of the above magnetic body can be suppressed, it becomes possible to improve the DC superposition characteristics.
Therefore, in the following Patent Literatures 1 and 2, as illustrated in FIG. 15, there is proposed an inductance element, in which a permanent magnet 22 is disposed inside a coil 21 buried in a magnetic body 20, and a magnetic flux X excited from the coil 21 is offset by a bias magnetic flux Y, which is emitted from the permanent magnet 22, and the direction of which is opposite to the direction of the magnetic flux X, so that the saturation of the magnetic body is suppressed, and the DC superposition characteristics is improved.
However, as illustrated in this figure, in a case where the permanent magnet 22 is disposed inside the coil 21, a leakage magnetic flux Z which does not act as a bias magnetic flux is emitted around the permanent magnet 22 inside the magnetic body 20, in addition to the magnetic flux Y whose direction is opposite to the direction of the magnetic flux X emitted from the permanent magnet 22 and excited by the coil 21. Therefore, there is a problem that the bias magnetic flux Y from the permanent magnet 22 does not effectively work, and the DC superposition characteristics can hardly be improved as expected.